Journal of Marine Science and Engineering Article Nearshore Dynamics of Storm Surges and Waves Induced by the 2018 Typhoons Jebi and Trami Based on the Analysis of Video Footage Recorded on the Coasts of Wakayama, Japan Yusuke Yamanaka 1,* , Yoshinao Matsuba 1,2 , Yoshimitsu Tajima 1 , Ryotaro Shibata 1, Naohiro Hattori 1, Lianhui Wu 1 and Naoko Okami 1 1 Department of Civil Engineering, The University of Tokyo, Tokyo 113-8656, Japan; [email protected] (Y.M.); [email protected] (Y.T.); [email protected] (R.S.); [email protected] (N.H.); [email protected] (L.W.); [email protected] (N.O.) 2 Research Fellow of Japan Society for the Promotion of Science, Tokyo 102-0083, Japan * Correspondence: [email protected] Received: 30 September 2019; Accepted: 11 November 2019; Published: 13 November 2019 Abstract: In this study, field surveys along the coasts of Wakayama Prefecture, Japan, were first conducted to investigate the coastal damage due to storm surges and storm-induced waves caused by the 2018 Typhoons Jebi and Trami. Special focus was placed on the characteristic behavior of nearshore waves through investigation of observed data, numerical simulations, and image analysis of video footage recorded on the coasts. The survey results indicated that inundation, wave overtopping, and drift debris caused by violent storm-induced waves were the dominant factors causing coastal damage. Results of numerical simulations showed that heights of storm-induced waves were predominantly greater than storm surge heights along the entire coast of Wakayama in both typhoons. However, computed gradual alongshore variations in wave and surge heights did not explain locally-concentrated inundation and run-up heights observed along the coasts. These results indicate that complex nearshore hydrodynamics induced by local nearshore bathymetry might have played a significant role in inducing such local wave characteristics and the associated coastal damage. Analysis of video footage recorded during Typhoon Jebi, for example, clearly showed evidence of amplified infragravity wave components, which could enhance inundation and wave run-up. Keywords: post-event survey; the 2018 Typhoons Jebi and Trami; tide gauge data; image analysis; infragravity wave 1. Introduction Tropical cyclone disasters are one of the major natural disasters in the world, as cyclone-induced winds, rain, waves, and surges cause catastrophic damage in urban areas (e.g., Hurricanes Katrina, Sandy, Irma, and Maria, and Typhoon Haiyan) [1–5]. Intense tropical cyclones, categorized into high-intensity classes (i.e., Categories 4 and 5), are expected to occur more frequently as global warming increases [6,7]. Therefore, individual damages induced by past tropical cyclones in each region should be thoroughly investigated to understand the mechanisms of damage generation and develop better disaster mitigation plans and strategies for the future. Japan has suffered major disasters induced by storms such as Typhoons Vera, Nancy, and Mireille [8–11]. Storm surges and storm-induced waves during a typhoon, as well as winds and rain, largely affect the country’s long coastlines. For example, Typhoon Vera, which crossed over the Kansai J. Mar. Sci. Eng. 2019, 7, 413; doi:10.3390/jmse7110413 www.mdpi.com/journal/jmse J. Mar. Sci. Eng. 2019, 7, 413 2 of 21 J. Mar. Sci. Eng. 2019, 7, x FOR PEER REVIEW 2 of 20 storm-surgeregion, the westerntyphoons part in Japan; of the mainthe maximum island of Japan, surge isheight known was as approximately one of the most 3.6 disastrous m at Nagoya storm-surge near thetyphoons head of Ise in Japan;Bay [12]. the Countermeasures maximum surge for height storm was surges approximately along the coast 3.6 m in at the Nagoya Kansai nearregion the have head beenof Ise discussed Bay [12]. using Countermeasures virtual typhoon for stormscenarios surges based along on the Typhoons coast in theVera Kansai and regionNancy have [12,13]. been Wakayamadiscussed Prefecture using virtual is one typhoon of the scenariosmost affected based areas on Typhoonsby storm surges Vera and and Nancy storm-induced [12,13]. Wakayama waves in thisPrefecture region because is one ofit has the long most coastlines affected areas facing by the storm open surges Pacific and Ocean, storm-induced extending for waves several in thishundred region kilometers.because it hasIn 2018, long coastlinestwo large facingtyphoons, the openJebi Pacificand Trami, Ocean, passed extending near forthe severalcoast of hundred Wakayama kilometers. and madeIn 2018, landfall two largeon the typhoons, prefecture Jebi and and generated Trami, passed extremely near thedamaging coast of storm Wakayama surges and and made waves landfall on the on coaststhe prefecture (Figure 1). and Many generated structures extremely and houses damaging near the storm coast surges were and inundated waves onand the damaged coasts (Figure during1 ). theseMany typhoons, structures as andreported houses by nearMori theet al. coast [13]. were The inundatedobserved heights and damaged of storm during surges theseat certain typhoons, tide gaugeas reported stations by inMori Wakayama et al. [13 ]were. The the observed highest heights in recorded of storm history surges during at certain either tide Typhoon gauge stations Jebi or in TyphoonWakayama Trami were [14,15]. the highest Consequently, in recorded these history typhoon durings and either the resulting Typhoon damages Jebi or Typhoon may have Trami been [14 the,15 ]. mostConsequently, disastrous theseevents typhoons on the andcoast the of resulting Wakayama damages in the may last have several been thedecades. most disastrousTherefore, events we investigatedon the coast coastal of Wakayama damage inin theWakayama last several induced decades. by these Therefore, two typhoons we investigated based on coastal a post-event damage fieldin Wakayama survey to inducedunderstand by thesethe overall two typhoons nearshor basede hydrodynamic on a post-event characteristics field survey induced to understand by storm the surgesoverall and nearshore storm-induced hydrodynamic waves. Additionally, characteristics in induced this study, by storm data analysis, surges and numerical storm-induced simulations, waves. andAdditionally, image analysis in this of study,the video data footage analysis, recorded numerical on the simulations, coasts were and performed image analysis to estimate of the them video duringfootage the recorded two typhons. on the coasts were performed to estimate them during the two typhons. Figure 1. Typhoon paths (indicated by lines) and surveyed locations (indicated by colored circles) in Figure 1. Typhoon paths (indicated by lines) and surveyed locations (indicated by colored circles) in Wakayama Prefecture. Red and blue indicate Typhoons Jebi and Trami, respectively. The typhoon paths Wakayama Prefecture. Red and blue indicate Typhoons Jebi and Trami, respectively. The typhoon were based on Japan Meteorological Agency, JMA [16]. The yellow squares indicate tide gauge stations paths were based on Japan Meteorological Agency, JMA [16]. The yellow squares indicate tide gauge at Kushimoto, Shirahama, Gobo, Wakayama, and Kobe from the south, and the yellow triangles indicate stations at Kushimoto, Shirahama, Gobo, Wakayama, and Kobe from the south, and the yellow Nationwide Ocean Wave information for Port and HArborS (NOWPHAS) stations at Shionomisaki, triangles indicate Nationwide Ocean Wave information for Port and HArborS (NOWPHAS) stations Komatsushima, and Kobe from the south. at Shionomisaki, Komatsushima, and Kobe from the south. J. Mar. Sci. Eng. 2019, 7, 413 3 of 21 2. Post-Event Surveys after the Typhoons 2.1. Damage Caused by Typhoon Jebi Typhoon Jebi passed near the coast of Wakayama during 11:00–13:00 JST (UTC + 9 h) on September 4, 2018, with a central pressure of approximately 950 hPa and the lowest central pressure dropping to 915 hPa [16] (Figure1). This typhoon caused a large disaster because of winds, rain, surges, and waves, not only on the coast of Wakayama but also along the coastal areas of Osaka Bay [13] (Figure1). Japan Society of Civil Engineers (JSCE) immediately organized a joint survey team, the 2018 Typhoon Jebi Coastal Disaster Survey Team, after the typhoon passed; this team surveyed the overall damage mainly on the coasts of Osaka, Hyogo, and Wakayama in the Kansai region [13,17]. Storm surges had a dominant influence near the inner part of Osaka Bay, with maximum inundation heights of + 4.0 and + 9.2 m, including wave run-up effects, from the mean sea level near the head of Tokyo Bay (i.e., Tokyo Peil, TP) [13]. The coasts of Wakayama were also severely damaged by storm surges and storm-induced waves during the typhoon. The authors conducted field surveys during 11 to 13 September and 19 to 21 September 2018, several weeks after the event, to determine the damage generated on the coasts. Horizontal coordinates and elevations of the observed drift-debris, destroyed structures, and witnessed watermarks, due to inundation and wave run-up, were measured by leveling from sea level and using a real-time kinematic global positioning system (RTK-GPS). The authors conducted this survey as part of the JSCE joint survey team, and most of the measured heights can be found in the dataset presented by Mori et al. [13]. The findings of this field survey after Typhoon Jebi are summarized as follows. The authors visited the coasts of Kushimoto, Susami, Shirahama, Tanabe, Minabe, Inami, Gobo, Mihama, Yura, and Arida (Figure1). Tide gauge stations, operated by Japan Meteorological Agency, JMA, at Kushimoto, Shirahama, and Gobo, observed peak surge levels of TP + 1.7 m, TP + 1.6 m, and TP + 3.2 m, respectively, at these locations during Jebi [13,14]. A substantial amount of debris was deposited onshore and covered most parts of the coast for several weeks after the typhoon. Figure2 shows debris deposited on a coastal dike with a crown height of TP + 9.2 m, observed at Tonda Beach in Shirahama; large amounts of debris were also found behind the dike.